Forgetting about 'foreach' for the moment, I'd like to talk about the
other end of things: opApply().
This code in D:
int opApply(int delegate(inout uint) dg)
{ int result = 0;
for (int i = 0; i < array.length; i++)
{
result = dg(array[i]);
if (result)
break;
}
return result;
}
seems to be rendered in Ruby-ish D as something like:
void opApply( ) |inout uint|
{
for (int i = 0; i < array.length; i++)
{
yield array[i];
}
}
Is there some way to make the real D one look as clean as the Rubified
one?
Main differences:
1) need to explicitly declare a delegate,
2) need to fiddle with return values.
For 1) I'm thinking if there were only some more compact way to declare
a delegate it would be golden.
How about this: with function literals, if you leave out the keyword
'delegate' or 'function' you get a delegate. I.e.
int delegate(long c) { return 6 + b; }
is equivalent to
int (long c) { return 6 + b; }
So it seems logical that as a variable declaration:
int delegate(long c) dg;
could also be made to be equivalent to
int (long c) dg;
In that case we could say in D:
void opApply(void (inout uint) dg)
{
for (int i = 0; i < array.length; i++)
{
dg(array[i]);
}
}
(assuming that pesky 'int return' business can also be sorted out).
The void can also be omitted frequently in D. So for a void-returning
delegate that could even be shortened to:
void opApply( (inout uint) block)
{
for (int i = 0; i < array.length; i++)
{
block(array[i]);
}
}
Now *that* is something I wouldn't mind staring at at 3am after a long
day coding. :-)
--bb

Shorter and more beautiful is good, and has merits indeed...
But sometimes clarity comes as a price, and that's not so good. If I
were new to D, this:
void opApply( (inout uint) block)
Would look like a funky sort of cast. That said, this:
void opApply( ) |inout uint|
Is at least clear, if not horrendously ugly to my eyes.
-[Unknown]

Forgetting about 'foreach' for the moment, I'd like to talk about the
other end of things: opApply().
This code in D:
int opApply(int delegate(inout uint) dg)
{ int result = 0;
for (int i = 0; i < array.length; i++)
{
result = dg(array[i]);
if (result)
break;
}
return result;
}
seems to be rendered in Ruby-ish D as something like:
void opApply( ) |inout uint|
{
for (int i = 0; i < array.length; i++)
{
yield array[i];
}
}
Is there some way to make the real D one look as clean as the Rubified one?
Main differences:
1) need to explicitly declare a delegate,
2) need to fiddle with return values.
For 1) I'm thinking if there were only some more compact way to declare
a delegate it would be golden.
How about this: with function literals, if you leave out the keyword
'delegate' or 'function' you get a delegate. I.e.
int delegate(long c) { return 6 + b; }
is equivalent to
int (long c) { return 6 + b; }
So it seems logical that as a variable declaration:
int delegate(long c) dg;
could also be made to be equivalent to
int (long c) dg;
In that case we could say in D:
void opApply(void (inout uint) dg)
{
for (int i = 0; i < array.length; i++)
{
dg(array[i]);
}
}
(assuming that pesky 'int return' business can also be sorted out).
The void can also be omitted frequently in D. So for a void-returning
delegate that could even be shortened to:
void opApply( (inout uint) block)
{
for (int i = 0; i < array.length; i++)
{
block(array[i]);
}
}
Now *that* is something I wouldn't mind staring at at 3am after a long
day coding. :-)
--bb

Shorter and more beautiful is good, and has merits indeed...
But sometimes clarity comes as a price, and that's not so good. If I
were new to D, this:
void opApply( (inout uint) block)
Would look like a funky sort of cast.

Well if you're new to D then there's a whole *lot* of things that are
going to look funky. Like foo!(int)() -- looks like some kind of
negation of a cast of nothing. Funky!

That said, this:
void opApply( ) |inout uint|
Is at least clear, if not horrendously ugly to my eyes.

I agree that it's not so ugly, but I don't think it's any clearer unless
you've been reading Ruby code. And it doesn't have any analogues
anywhere else in D. At least leaving out the keyword 'delegate' and the
keyword 'void' is already sanctioned practice elsewhere in D. Being
able to leave it out on variable declarations actually makes things
*more* consistent.
--bb

I know Ruby, and it's loops are very cool, indeed. But this is D, and as
a full blown *system and application programming language*, you can't
compare it to Ruby. I don't think syntactic sugar should be added too
hasty. Apart from that I kinda like the delegate / function ptr syntax
as it is.
Alex
On Fri, 2006-10-20 at 12:41 +0900, Bill Baxter wrote:

Forgetting about 'foreach' for the moment, I'd like to talk about the
other end of things: opApply().
This code in D:
int opApply(int delegate(inout uint) dg)
{ int result = 0;
for (int i = 0; i < array.length; i++)
{
result = dg(array[i]);
if (result)
break;
}
return result;
}
seems to be rendered in Ruby-ish D as something like:
void opApply( ) |inout uint|
{
for (int i = 0; i < array.length; i++)
{
yield array[i];
}
}
Is there some way to make the real D one look as clean as the Rubified
one?
Main differences:
1) need to explicitly declare a delegate,
2) need to fiddle with return values.
For 1) I'm thinking if there were only some more compact way to declare
a delegate it would be golden.
How about this: with function literals, if you leave out the keyword
'delegate' or 'function' you get a delegate. I.e.
int delegate(long c) { return 6 + b; }
is equivalent to
int (long c) { return 6 + b; }
So it seems logical that as a variable declaration:
int delegate(long c) dg;
could also be made to be equivalent to
int (long c) dg;
In that case we could say in D:
void opApply(void (inout uint) dg)
{
for (int i = 0; i < array.length; i++)
{
dg(array[i]);
}
}
(assuming that pesky 'int return' business can also be sorted out).
The void can also be omitted frequently in D. So for a void-returning
delegate that could even be shortened to:
void opApply( (inout uint) block)
{
for (int i = 0; i < array.length; i++)
{
block(array[i]);
}
}
Now *that* is something I wouldn't mind staring at at 3am after a long
day coding. :-)
--bb

I know Ruby, and it's loops are very cool, indeed. But this is D, and as
a full blown *system and application programming language*, you can't
compare it to Ruby.

What is D if not "the speed of C++ with the ease of Ruby"?

I don't think syntactic sugar should be added too
hasty.

I agree. That's why plenty of discussion is needed. So start poking holes!

Apart from that I kinda like the delegate / function ptr syntax
as it is.

And you could still use it as is if you like, because the suggestion is
to make 'delegate' optional, not mandate it's removal.
About whether sugar is warranted here: it seems Walter sees this
delegate mechanism as becoming *the* primary technique for iteration in
D. That being the case, it should be as easy to read and write as
possible. Being easier to use than C++ iterators is the primary reason
he gives for liking it, in fact. So let's figure out how to make
absolutely as simple as possible.
Right now, say you want to write a generic array iterator factory
function 'traverser', which you do because it allows you to do this:
foreach(int i; iarray.traverser()) {
...
}
to iterate in some custom way over the elements of any array.
Here is the signature for that now (returns a delegate that takes a
delegate parameter):
int delegate(int delegate(inout typeof(ArrayT[0])))
traverser(ArrayT)(inout ArrayT array)
{
}
...
The int delegate(int delegate ...) business is just too verbose to grok
easily. If I change the ints to voids and drop the 'delegate':
(( inout typeof(ArrayT[0]) )) reversed(ArrayT)(inout ArrayT array)
I think that's easier to look at if for no other reason than being
shorter. But I'll admit it probably is more mind boggling at first.
But it's not a stretch to say most everybody could get used to reading
(int) somefunc() {...}
as a function that returns a void-returning, int-taking delegate. Just
think of the (int) as a lone argument list ["takes an int"] and it's
pretty clear.
And in that light it's not hard to see the extra sets of parens (( ))
mean 'a void returning delegate that takes a void returning delegate'.
At some point (( type )) just becomes second nature as the basic return
signature for an iterator factory. It's easy to see once you know to
look for it, because its so short and (( )) stands out if spaced properly.
Another alternative is standard aliases for those complicated types:
template Types(ArgT) {
static if( is (ArgT[0]) ) {
alias typeof(ArgT[0]) elem_t;
}
else static if( is(ArgT.elem_t) ) {
alias ArgT.elem_t elem_t;
}
else {
alias ArgT elem_t;
}
alias int delegate(int delegate(inout elem_t)) iter_t;
// this works for anything that is either array-like or has an
// elem_t alias.
}
Then you can use those aliases like:
Types!(ArrayT).iter_t reversed(ArrayT)(inout ArrayT array)
{ ... }
that still looks kinda klunky :-/, but maybe better. At least it's
short enough to fit on one line. But it just replaces having to know
what (( )) means with having to know what Types!().iter_t means. So I'm
not sure it's really better.
Well at least it is something that works now!
--bb

I know Ruby, and it's loops are very cool, indeed. But this is D, and as
a full blown *system and application programming language*, you can't
compare it to Ruby.

leads on to D being more suitable for compilation than Ruby, and thus
its speed, type-safety and meta-programming, and D not as concisely
handling some aspects of reflection. Other than that, they are the
leading languages in their fields (interpreted and compiled languages),
and can learn a lot from each other.
Cheers,
Reiner

The proposal looks good (as long as it creates no syntactical
ambiguities) but if we had variadic template parameters (see
http://www.generic-programming.org/~dgregor/cpp/variadic-templates.html
for a good version for C++) then you could do it much more easily with
templates:
template IterFunc(type ...)
{
alias int delegate(type ...) IterFunc;
}
and then you could just declare your iterators:
int opApply(IterFunc!(int) dg)
The best thing about this is you can easily modify it:
template IndexedIterFunc(type ...)
{
alias int delegate(size_t index, type ...) IndexedIterFunc;
}
or if the return status was instead the first function parameter (to
allow for return values, not void):
template NewIterFunc(retVal, type ...)
{
alias retVal delegate(out int status, type ...) NewIterFunc;
}
Unfortunately, this wouldn't handle 'inout' parameters, since you can't
instantiate a template with an inout parameter...
Cheers,
Reiner

Ooh, I missed this "Implicit Template Properties" thing. That's an
improvement over what I had even without the variadic bit. Just means I
need an IterFunc1, IterFunc2,... etc.

and then you could just declare your iterators:
int opApply(IterFunc!(int) dg)
[...]
Unfortunately, this wouldn't handle 'inout' parameters, since you can't
instantiate a template with an inout parameter...

Ah, that's a pity. Is that a spec limitation or just something still on
the "todo" list?
--bb

Forgetting about 'foreach' for the moment, I'd like to talk about the
other end of things: opApply().
<snip>

You could always use this handy template which automatically exposes any
array or assoc-array in a class you want, with a single mixin.
class Foo
{
Bar[] bar;
mixin applyThis!(Bar,array);
}
Bar b = new Bar();
foreach( Bar b; foo ) {...}
-DavidM

Forgetting about 'foreach' for the moment, I'd like to talk about the
other end of things: opApply().
<snip>

You could always use this handy template which automatically exposes any
array or assoc-array in a class you want, with a single mixin.
[...]

Wow. That is very cool. D is spiffy. :-)
But I still think the core syntax should be clean.
Makes me wonder, though, if C++ had mixins like that, maybe proper
iterators wouldn't be so hard to do correctly there either? Just a
matter of adding in a line:
iteratorThis!(...)
--bb

The compiler knows the type of array[i], so it can infer what the type
of the parameter is. This inference should never be wrong, as long as
the delegate is called at least once.
The return type can't be inferred, however, so we would combine your
syntax with type inference to get some hybrid like this:
int opApply(int (auto) dg)
To handle inout parameters, you simply need inout to be allowed at the
call-site:
result = dg(inout array[i]);
C# requires (!) this, and several people have requested that this be
optional in D -- why shouldn't it?
The only problem I can see with this is that it moves the type of the
delegate out of the function prototype and into the function. Never mind
-- many people from functional languages with Hindley-Milner type
inference have been doing that for years and not had problems with it;
the saving is more than the gain.

I know Ruby, and it's loops are very cool, indeed. But this is D, and as
a full blown *system and application programming language*, you can't
compare it to Ruby.

What is D if not "the speed of C++ with the ease of Ruby"?

D has a C like syntax, Ruby does definitely not. :)

I don't think syntactic sugar should be added too
hasty.

I agree. That's why plenty of discussion is needed. So start poking holes!

Apart from that I kinda like the delegate / function ptr syntax
as it is.

And you could still use it as is if you like, because the suggestion is
to make 'delegate' optional, not mandate it's removal.

Wouldn't it be better to solve such things with a little library (as you
suggested)? I do like the constructs of higher level languages that have
been added to D, but the meaning of each should be recognizable on the
first sight. So when you look at a big fat nested arguments list you'd
use your syntax to shorten it - but would it be readable for others?
What D has made possible [imho] is to make code more readable. You have
explicitly named or styled syntax sugar, that is mostly not used as is
in other languages, and thus can't really be misread through confusion.
I wouldn't want this to be changed, actually.

About whether sugar is warranted here: it seems Walter sees this
delegate mechanism as becoming *the* primary technique for iteration in
D. That being the case, it should be as easy to read and write as
possible. Being easier to use than C++ iterators is the primary reason
he gives for liking it, in fact. So let's figure out how to make
absolutely as simple as possible.

alias! :P

Right now, say you want to write a generic array iterator factory
function 'traverser', which you do because it allows you to do this:
foreach(int i; iarray.traverser()) {
...
}
to iterate in some custom way over the elements of any array.
Here is the signature for that now (returns a delegate that takes a
delegate parameter):
int delegate(int delegate(inout typeof(ArrayT[0])))
traverser(ArrayT)(inout ArrayT array)
{
}
...
The int delegate(int delegate ...) business is just too verbose to grok
easily. If I change the ints to voids and drop the 'delegate':
(( inout typeof(ArrayT[0]) )) reversed(ArrayT)(inout ArrayT array)
I think that's easier to look at if for no other reason than being
shorter. But I'll admit it probably is more mind boggling at first.
But it's not a stretch to say most everybody could get used to reading
(int) somefunc() {...}
as a function that returns a void-returning, int-taking delegate. Just
think of the (int) as a lone argument list ["takes an int"] and it's
pretty clear.
And in that light it's not hard to see the extra sets of parens (( ))
mean 'a void returning delegate that takes a void returning delegate'.
At some point (( type )) just becomes second nature as the basic return
signature for an iterator factory. It's easy to see once you know to
look for it, because its so short and (( )) stands out if spaced properly.
Another alternative is standard aliases for those complicated types:
template Types(ArgT) {
static if( is (ArgT[0]) ) {
alias typeof(ArgT[0]) elem_t;
}
else static if( is(ArgT.elem_t) ) {
alias ArgT.elem_t elem_t;
}
else {
alias ArgT elem_t;
}
alias int delegate(int delegate(inout elem_t)) iter_t;
// this works for anything that is either array-like or has an
// elem_t alias.
}
Then you can use those aliases like:
Types!(ArrayT).iter_t reversed(ArrayT)(inout ArrayT array)
{ ... }
that still looks kinda klunky :-/, but maybe better. At least it's
short enough to fit on one line. But it just replaces having to know
what (( )) means with having to know what Types!().iter_t means. So I'm
not sure it's really better.

I doesn't look klunky to me, actually. You can still use standard
aliases to 'beautify' it, for example if you make a library of such
things.